Dual carb question, Dellorto 40s on a 1.8L to be exact Options

[terrymason]

The problem - The car will go very lean around 2500 rpm, as if the pump jets were too small.

History:
New fuel pump (3.5 PSI to each carb)
55 idle jets
55 pump jets
140 main jets
175 air correctors
These carbs are new to the car, and I'm trying to dial them in.

The car is lean when I first start it (16 / 17 on my air / fuel gauge), but then levels out to a nice 14.7 idle when warmed up. While driving, I can give the car 20% or so of the pedal, and drive up to highway speeds nicely, but when I goose it, it will go very lean (18-22 on my gauge) followed by a series of backfires and loss of power. I started off with 35 pump jets, and have moved up to 55s, which appear to be the most popular around here. I've also adjusted the pump jet linkage, from the nut fully loosened, and moved it in as far as it would go before affecting the main throttle shaft (like 60 turns or something).

What do you guys think, do I need 75 pump jets?

[terrymason]

OK, I have made some big headway on this. I pulled all the jets from the car, and blew them out with carb cleaner, along with the places they go. Replaced my 52 idle jets with 60s.

I think cranked the idle up to 2500, and used my unisyn gauge to check the flow. The passenger was a 5, while the driver was at 12. I adjusted the linkage so that they are both at 11 around 2500 rpm.

I then addressed my mixture, making all screws 2.5 turns out.

I then adjusted my pump jets. turning them about midway out. They were previously all the way tightened (I was trying to get a larger pump squirt)

I finally closed all my air bypass screws.

Now, the car idles around 10.5 a/f (probably from the 60 idle jets). It drives 100% better, but still goes lean when it starts to pull (2nd gear feels it most). It also seems that if I goose the gas it will go lean.

[type11969]

Did you ever check out the vent size?

You should not be adjusting the linkage to balance the carbs, the carbs have to be balanced with the linkage disconnected. Once balanced, the linkage gets reconnected, and you try to make sure both carbs open and close at the same time and at the same rate.

Once at highway rpms, are you still running lean? Or only when accelerating? If it is the latter, it could be due to oversized vents but you may be able to correct for it with the pump shot. Verify your vent size, consider jumping up to 180 airs, and check your e-tubes. You also should try to measure your pump volume, you should gun for at least .2 ccs per two pumps.

The smallest vents CB had as of a month ago were 32s, they will be better than 34s for you (if you actually are running 34s).

-Chris

OK, I have made some big headway on this. I pulled all the jets from the car, and blew them out with carb cleaner, along with the places they go. Replaced my 52 idle jets with 60s.

I think cranked the idle up to 2500, and used my unisyn gauge to check the flow. The passenger was a 5, while the driver was at 12. I adjusted the linkage so that they are both at 11 around 2500 rpm.

I then addressed my mixture, making all screws 2.5 turns out.

I then adjusted my pump jets. turning them about midway out. They were previously all the way tightened (I was trying to get a larger pump squirt)

I finally closed all my air bypass screws.

Now, the car idles around 10.5 a/f (probably from the 60 idle jets). It drives 100% better, but still goes lean when it starts to pull (2nd gear feels it most). It also seems that if I goose the gas it will go lean.

[BxtrBill]

[/quote]The smallest vents CB had as of a month ago were 32s, they will be better than 34s for you (if you actually are running 34s).

The vents for the 36mm Dells also fit the 40s (same carb body), and you can get them down to 28mm at least. Just a thought. Can't say if CB has them, though.

Oh..almost forgot.....if you look down the carb throat at the top edge of the venturi, you'll see a number that indicates the venturi size.

[terrymason]

Oh..almost forgot.....if you look down the carb throat at the top edge of the venturi, you'll see a number that indicates the venturi size.

Are you sure, or is it possible to put the in upside down, so that the numbers are at the bottom? I took a flashlight and stared at them for about 5 minutes and couldn't see anything.

[BxtrBill]

Are you sure, or is it possible to put the in upside down, so that the numbers are at the bottom? I took a flashlight and stared at them for about 5 minutes and couldn't see anything.

Hm..hadn't considered someone might put them in upside down, but it's entirely possible to do so. I can't say what to expect from an upside-down venturi, but I'd be fairly certain the carb(s) wouldn't work as designed....I'd guess it would upset airflow by moving the low-pressure area of the venturi lower in the carb, but I never tried it so don't know what to expect. Before you do anything else, it may be worthwhile to check one more time for the numbers.....they're small, but they *should* be there. If they're not, someone has reversed the vents in the carb body, maybe the same guy who "drilled out" the emulsion tubes (hint: don't ever do that) Emulsion tubes, in concert with the air correction and main jets, control mixture in relation to airflow. E-tubes are available to lean or richen the mixture at different spots in the rev range, but generally you're best off with the tubes that came with the carbs. You should be fine with 180 airs, too....you can go to 200s and that will have the effect of leaning the mixture slightly across the range, but to no real advantage.

It's easy to remove, then reinstall the vents correctly. Remove the air cleaners and the top cover of the carburetor. If you're really brave, do this while the carb is still on the engine. I'm not that brave, so I do this on a bench. There is a set screw with locknut at each end of the carb body....loosen the locknut, then the setscrew, and remove the aux (boost) venturis, then the pump jets. If the carbs have been cleaned recently, the main venturis should slide right out...if not, they may take a bit of gentle persuasion. Nothing drastic, please...they're just potmetal. Stick 'em back in with the numbers toward the top side of the carb, and screw everything back together. Or as the Haynes says, "Assembly is the reverse of disassembly"

One thing I'd add.....when you're trying to troubleshoot something like this, Change One Thing At A Time. Focus on one part of the problem...the idle....midrange, transition, high-speed running, acceleration, etc, insofar as possible. Carb functions overlap, granted, but it's a real booger to guage the effects (if any) of a change when two or three things changed at once. Ok...off the soapbox now, and you probably knew that anyway.